IPT systems are finding increasingly wide application, including small devices such as electric toothbrushes and higher power applications such as electric car charging.
A typical IPT system consists of two main blocks, the power transmitter (hereinafter “transmitter”) and power receiver (hereinafter “receiver”) which form a unidirectional power train using the B-field coupling. The transmitter usually includes a converter driving a transmit winding/coil at a transmitter frequency typically between 10 kHz and 1 MHz The receiver has a receiver winding/coil, and may include a receiver converter which converts the transmitter frequency to DC. In other arrangements the receiver converter may provide an AC output.
The transmitter frequency may be fixed but more usually varies, for example due to load conditions, environmental conditions, and coupling conditions.
Losses occur in a number of situations but specifically where the receiver operates at a frequency substantially different to the transmitter frequency. Where a transmitter is resonant, some prior art mitigates this by using a relatively flat (i.e. relatively low Q) resonant characteristic.
FIG. 1 shows two illustrative resonant curves, Qx and Qy, with Qy being high Q relative to Qx. These are plotted against power transfer, W. Both curves have a resonant frequency of fr and it will be seen that operating at a frequency f1 being in this illustrative example a lower frequency than fr causes a relatively small loss in power Δ1 on Qx. On the other hand for the high Q situation operating at the frequency f1 causes a much larger loss in power Δ2 on Qy.